For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive (vertical) load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile (horizontal) forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as 100 feet can be attained in members as deep as three feet for roof loads. The basic principal is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principal, but the reinforcing is held loosely in place while the concrete is placed around it. The reinforcing is then stretched by hydraulic jacks and securely anchored into place. Prestressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there is provided a pair of anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the anchor suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of byproducts from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
Several U.S. patents have considered the problem of anchor and tendon corrosion. U.S. Pat. No. 4,348,844, issued to Morris Schupack et al., on Sep. 14, 1982, disclosed a tendon assembly in which a tendon is enclosed in a sheath suspended under tension between two spaced anchor members. The anchor members are entirely enclosed within an envelope or a housing. The sheath, the envelope, and the housing are required to comprise electrically non-conductive materials for electrically isolating the tendon and anchor members from a surrounding concrete structure to thereby prevent the effects of electrolysis caused by electrical currents.
U.S. Pat. No. 4,616,458, issued to Davis et al., on Oct. 14, 1986, provided a plastic structure for protecting the anchor assembly and the ends of a tendon from exposure to the corrosive elements. The system of this patent describes a protective top member and a protective bottom member. The anchor was interposed between these members, the members were snap-fitted together, and the anchor locked into position between these protective members. Grease was then injected into the interior between these protective plastic members so as to seal the anchor from the corrosive water in the environment. A grease cap would be threaded onto the protective top member so as to allow grease to be injected into the interior space.
U.S. Pat. No. 4,896,470 issued on Jan. 30, 1990 to the present inventor, describes a tendon tensioning anchor which includes a base member having a tubular extension extending therefrom and aplastic encapsulation in generally juxtaposition with the exterior of the base member and the exterior of the tubular section. The plastic encapsulation includes a polymeric material of high density polyethylene and in injection-molded relationship to the base member and the tubular section. The encapsulation is of a unitary construction. The base member and the sloping annular interior wall of the tubular section are in contact with the tendon extending therethrough so as to establish an electrolytic contact. The tubular section has a sloping annular interior wall for receiving the end of the tendon.
U.S. Pat. No. 5,072,558, issued on Dec. 17, 1991 to the present inventor, also describes a post-tension anchor system having a similar construction to that of U.S. Pat. No. 4,896,470. U.S. Pat. No. 5,072,558 describes the use of a heat shield fastened within the polymer encapsulation adjacent an end of the tubular section that extends outwardly of the base. The polymer encapsulation includes a tubular portion formed at a side of the base member opposite the tubular section and extends outwardly perpendicular to the base member. The heat shield is a rigid member having an outer diameter corresponding to the outer diameter to the tubular section. Extension tubing is fitted to the end of the tubular section of the polymer encapsulation. A seal is fastened within the other end of the extension tubing so as to create a liquid-tight seal with a tendon passing therethrough.
FIG. 1 shows a prior art system manufactured in accordance with the teachings of U.S. Pat. Nos. 4,896,470 and 5,072,558. This system is presently sold by General Technologies, Inc., of Stafford, Tex. As can be seen in FIG. 1, the base member 10 has a polymeric encapsulation 12 extending thereover. A tubular section 14 extends outwardly of the base member 12 generally centrally thereof. Flanges 16 and 18 extend outwardly from opposite sides of the tubular section 14. The polymeric encapsulation 12 will extend around the base member 10 and over and around the tubular section 14. A pair of nail holes 20 and 22 are formed through the base member 10 so as to allow the base member 10 to be secured to an exterior surface. A cap 24 is illustrated as suitable for connection over the end 26 of tendon 28 so as to maintain the end 26 of tendon 28 in a generally air-tight and liquid-tight environment. A suitable grouting material can fill the interior of the cap 24 so as to further avoid any contaminating effects from the exterior environment as affecting the tendon 28. A tubular section 30 extends outwardly from the opposite side of the base member 10 opposite the tubular section 14. A sealing tube 32 is illustrated as positioned for friction-fit relationship to the tubular section 30. Tendon 28 will extend through the sealing tube 32. A sheathing 34 is applied over the exterior of the tendon 28 so as to further avoid corrosive effects from the exterior environment. A seal 36 engages with the sheathing 34 so as to maintain the tendon 28 in a generally liquid-tight and air-tight environment.
FIG. 2 is a cross-sectional view of the anchor system as illustrated in FIG. 1. In particular, it can be seen that the base member 10 is a steel anchor of a generally conventional configuration. The tubular section 14 has a sloping annular interior wall 38. The polymeric encapsulation 12 will extend over the surfaces of the tubular section 14 and over the flanges 16 and 18 of the base member 10. Tubular portion 30 is illustrated as extending in longitudinal alignment with the tubular section 14. The polymeric encapsulation includes a portion 40 which extends over and around the exterior of the tubular section 14 and also defines the cap-receiving receptacle 42. The polymeric encapsulation 12 is integrally formed over the base member 10 and serves to define the portion 40 and the tubular portion 32.
In use of the anchor described in FIGS. 1 and 2, it was found that a certain amount of polymeric shrinkage can occur over time. The shrinkage of polymers is a natural condition of polymers. Whenever the polymeric encapsulation 12 should shrink around the base member 10, surfaces 44 and 46 will tend to pull away from the surfaces 48 and 50 of the base member 10. When the polymeric encapsulation tends to pull away from surfaces 48 and 50, several problems can result. First, the polymeric encapsulation 12 will no longer be in surface-to-surface with the surfaces of the base member 10. As such, it is possible that air and liquid intrusion can occur into such spaces. Secondly, the forces imparted by the shrinking polymeric encapsulation 12 could potentially create sufficient forces so as to cause a fracture of the steel material used for the base member 10. This can especially be the case where a certain leverage effect is created by the force exerted by the shrinking polymer material upon the far ends of the outwardly extending flanges 16 and 18. In other words, when the polymeric encapsulation 12 should shrink, the pulling forces will extend between the tubular section 14 and the very ends of the flanges 16 and 18. As such, it is very important to be able to provide a system whereby such polymer shrinkage will not adversely affect the integrity of the post-tension anchoring system.
It is an object of the present invention to provide a post-tension anchor system which effectively avoids any adverse effects caused by polymer shrinkage.
It is another object of the present invention to provide a post-tension anchor system which maintains the polymeric encapsulation in strong surface-to-surface contact with the surfaces of the anchor.
It is a further object of the present invention to provide a post-tension anchor system which minimizes potential fractures of the steel base member.
It is a still another object of the present invention to provide a post-tension anchor system which is easy to manufacture, easy to use and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from the reading of the attached specification and appended claims.